Unlike more traditional semiconductor components, a “microelectromechanical systems” (“MEMS”) transducer by definition includes a “mechanical” component. By way of example and not limitation, a MEMS transducer of the present invention can be a pressure sensor, microphone, accelerometer, gyroscope, chemical sensor, or other property measurement devices, each of which requires movement: A pressure sensor may have a deformable diaphragm that incorporates piezoresistive material that will undergo electrical conductivity changes as the diaphragm deforms. A microphone may have a fixed backplate and deformable diaphragm that form the plates of a capacitor, with the capacitance changing in response to the amplitude and frequency of the sound waves striking the diaphragm. An accelerometer may have a movable spring-loaded plate in close proximity to a fixed plate, such that when the accelerometer experiences inertial motion—acceleration, deceleration, or rotation—the capacitance between the plates will change. In a chemical sensor, the device itself doesn't move, but instead, may have ports that allow the movement of fluids through the sensor; as the surface of the device absorbs fluids passing through the sensor, the electrical characteristics of the sensor (for example, its conductivity) changes. As such, a package for a MEMS transducer must provide the traditional elements of a semiconductor package—protection from physical damage and electromagnetic interference, first and second-level interconnections, geometric translation, thermal management, and the like—but it must also provide interior space where the mechanical components of the MEMS transducer have freedom to move.
A common MEMS package in the prior art may involve mechanically and electrically coupling the MEMS transducer to a flat substrate and then attaching a cup-shaped lid over the top. For example, U.S. Pat. No. 6,781,231 discloses a MEMS microphone package comprising a MEMS transducer attached to a PCB substrate and covered by a cup-shaped metal cap. Such MEMS packages can be manufactured in quantity by attaching multiple MEMS transducers on a PCB panel, attaching caps over the MEMS transducers, and then dicing the panel into individual MEMS packages, as disclosed, for example, by U.S. Pat. No. 8,018,049.
However, manufacturing MEMS packages according to prior art has certain drawbacks. First, as shown in FIG. 1, a cup-shaped cap may have rounded edges on the top of the package; in some applications, it is preferable to have a MEMS package with sharp 90° edges on all corners, such as the package shown in FIG. 2. Thus there is the need for a package and manufacturing method that utilizes a rounded cap but results in a package with sharp corners.
Second, for many applications, it is sufficient to mount the transducer to the same side of the package as the second-level interconnects. However, in some applications, it is preferable to mount the transducer to the opposite side of the package as the second-level interconnects. For example, a top-port surface mount MEMS microphone may have more-favorable acoustic characteristics when the MEMS microphone die is mounted directly over the port. In the prior art, this could be achieved by putting the port in the substrate, mounting the die over the port, putting the second-level interconnects on the cover, and including electrical pathways in the walls of the cover. However, because the package cavity must be acoustically sealed, a seal must be included between the cover and substrate; in the prior art, as shown in FIGS. 21 and 22, the seal was achieved by using a continuous solder seal ring 402 around the perimeter of the substrate 400 with a corresponding solder ring 403 on the bottom of the cover 401, However, this approach requires more space and is susceptible to shorts between the solder rings and the pads. Thus, there is a need for a package and manufacturing method that seals the cap to the substrate without the need for a solder ring.
Embodiments disclosed and claimed herein address these issues and others that will be clear to one of ordinary skill.